After removing coal or similar aggregate materials from a mine or strip pit, it is necessary to further process the material, referred to as run-of-mine (ROM) coal, by breaking it, sorting it into certain sizes and removing rock, shale or other impurities. ROM coal is typically delivered to a breaker, which is a large cylindrical shell with interior lifting blades or shelves. The shell of the breaker is perforated with holes to permit passage of small material. The breaker rotates on a horizontal axis, receiving material in one end, tumbling it as it passes thorough the holes in the shell, and permitting the hard, large, unbroken material to pass out the rear of the machine through a discharge spiral or auger. The coal exiting the shell through the holes is processed further, while the large rejected material is hauled away as waste. The “Bradford Breaker” is one example of a commonly used rotary breaker.
Rotary breakers have seen little improvement and undergone few changes since their introduction in the 1870's. While the demand for increased productivity at mining operations continues to grow, designers and manufacturers of these devices have made no large-scale improvements to their design. Typically, the only change in the process made by manufacturers to meet the increasing maximum feed rate demands is simply to increase the size of the apparatus.
As the total amount of material that enters a rotary breaker increases and approaches the design limits of the apparatus, the efficiency of the breaker decreases and allows desirable product to exit the breaker through the spiral discharge at the rear of the machine as waste. To accommodate greater maximum feed rates, the diameter and length of the rotary breaker can be increased. But the increase in maximum feed rate comes at the expense of efficiency at lower feed rates. As the feed rate of material entering the breaker decreases, the material within the breaker is subjected to excessive breakage. Rock and other impurities reach sizes that are permitted to exit the breaker through the holes in the shell rather than through the discharge spiral at the rear of the machine. Further, the coal is also over processed and reduced to undesirable fines.
Accordingly, there is a need for an improved rotary breaker design that not only permits the efficient processing of coal at greater maximum feed rates, but also allows for efficient operation over a wide range of feed rates without sacrificing product quality or increasing the waste of saleable product.